Police are investigating a sensational claim that the SAS was involved in the death of Princess Diana, reports Sean Rayment.

The allegation emerged at the second court martial of Sergeant Danny Nightingale, who was found guilty of illegally possessing a gun and ammunition.

It came in a letter to the elite unit’s commanding officer by the parents-in-law of a special forces sniper, known only as Soldier N, who was Sgt Nightingale’s former housemate and a key witness for the prosecution.

The Ministry of Defence, the Royal Military Police and the Service Prosecuting Authority have known of its existence and the claim since it was sent in September 2011.

The seven-page hand-written letter – a copy of which has been seen by the Sunday People – also makes allegations over Soldier N’s behaviour towards his wife and her family following the collapse of the couple’s marriage.

It is understood that the SAS passed the letter to the Service Prosecuting Authority prior to the start of the Sgt Nightingale trial.

All references to the paramilitary force were removed by the SPA before it released the document to the court.